The present invention relates to ion sources for an ion implanter, an ion beam writing apparatus, etc., and more particularly to a liquid metal ion source from which the ion species of a high vapor pressure element P can be extracted as the ions of the single element efficiently and stably for a long time.
As performances have been enhanced in the fields of micro-dry processes (such as ion beam exposure, dry developement and microdoping), a submicron surface analysis, etc., it has been eagerly desired to develop a liquid metal ion source of high brightness. Particularly in the field of electro-devices, it has been attempted to use liquid metal ion sources from which the ions of Ga, Si, Be etc. being dopant species can be extracted, and to directly dope (implant) semiconductor materials with the dopants.
The operating principle of these liquid metal ion sources is as follows. First, an emitter made of tungsten W, tantalum Ta, carbon C, or the like is supplied with a source feed material (a material which contains the element of a desired ion beam and which shall hereinafter be termed a "source material") melted by resistance heating or by electron-beam bombardment, laser-beam irradiation or the like. Secondly, an intense electric field is applied to the tip of the emitter, and the ions of the source material are extracted from the emitter tip by field ionization. Accordingly, it is important for the ion source that the desired ion beam can be extracted stably for a long time.
Meantime, one of the most important elements among n-type impurity elements for a silicon semiconductor is P, while one among p-type impurity elements is B. There has not hitherto been any concrete report on the emission of P ions from a liquid metal ion source.
The P element has a melting point of 44.1.degree. C., at which its vapor pressure is as high as 0.181 mmHg, and hence, it is difficult to use the P element as the source material of a liquid metal ion source. Therefore, a method becomes effective in which the difficulty is relieved in the form of an alloy composed of P and a metal or metals other than P, a liquid metal containing P is formed under the condition of a high temperature, the ions of the alloy constituting elements are extracted using the alloy as a source material under the condition of a high electric field, and only P ions are separated and obtained by mass separation. It is accordingly an important point to search for the source material which contains P, namely, the alloy from which the P ions can be extracted stably and for a long time.
The Official Gazette of Japanese Laid-open Patent Application No. 57-132653 discloses a source material which is made of an alloy composed of a transition element and at least one metalloid element selected from the group consisting of Al, As, B, C, Ge, In, P, Si and Sn. However, the transition elements concretely mentioned include only Ni, Pd, Pt, La, Au and Fe. As will be described later, a source material in a liquid metal ion source according to the present invention is a Cu-P-based alloy. The aforementioned official gazette contains quite no statement indicating that this alloy is especially excellent. In order to show the state of the art of the present invention, besides the aforementioned official gazette, (i) the Official Gazette of Japanese Laid-open Patent Application No. 57-30243 and (ii) the Official Gazette of Japanese Laid-open Patent Application No. 58-178944 can be mentioned.